When laser drilling holes or in other manufacturing processes, the machining tool must be positioned accurately with respect to the part being manufactured if the final part is to be made with any precision. In laser drilling, for example, the laser nozzle may be positioned in a nominal drilling location relative to a nominal surface of the part, the nominal drilling location being determined solely using a nominal computer model of the part, provided for example by a CAD (computer aided design) file. However, such an approach does not provide feedback as to whether the actual part is in the desired nominal location or has the desired nominal surface. Because of the differences between the actual surface and the nominal computer model of that surface, the holes may inadvertently be drilled at different locations and/or angles than those desired and defined in the nominal model, and as such may also have a diameter different from the nominal diameter. Thus, the final part may not be as desired. In the case of gas turbine parts in which cooling holes are drilled, the airflow rate and its direction through the cooling holes for the actual part may be different than for the nominal part, which can reduce the cooling effectiveness and as such the life of the actual part.
FIG. 1A shows an example where a nominal straight profile as defined by the nominal computer model is indicated at N, and actual straight profiles located respectively inwardly and outwardly of the nominal profile N are indicated at I and O. A laser nozzle D is moved along its axis such as to keep a given distance SD with the actual surface of the part, and the nozzle D is aligned with the hole location of the nominal profile N. It can be seen that the hole locations in the actual profiles I, O, are different from one another and from the hole location set forth in the nominal profile N. As such, an undesirable variation d is introduced in the location of the hole depending on the actual profile of the part.
FIG. 1B shows another example where actual, arcuate profiles I and O are located respectively inwardly and outwardly of an arcuate nominal profile N. In this case, as the nozzle D is aligned with the hole location of the nominal profile N, the variation between the nominal profile N and the actual profiles I and O not only introduces a variation in the location of the hole, but also a variation between the hole angle θN defined in the nominal profile and the hole angles θI and θO produced in the actual profiles I and O, with θI<θN<θO. In addition, the drilling operation can also produce a back strike B when drilling the hole, thus damaging the interior surface of the part.
Similar variations between machined elements in an actual part and those same elements in a nominal part can also be observed in a variety of machining or other manufacturing operations.
Accordingly, there is a need to provide improved compensation for the differences between the nominal definition of a part and the actual part.